The present invention relates to angular rate sensors for sensing rotation in inertial space. More specifically the invention relates to ring laser gyroscopes which operate in radiation environments.
Ring laser gyroscopes are used as inertial sensors in many applications including all types of navigation systems. Generally speaking, ring laser gyroscopes operate by supporting two counterpropagating optical signals within a closed loop path. An ultrastable block containing a closed loop cavity is used to support the counterpropagating optical signals. As this block is rotated, a frequency separation is created between the two optical signals which is indicative of rotation. This separation is then detected and used as a measurement of inertial rotation.
The two optical signals are generally created by generating a gas discharge laser within a cavity. One type of gas discharge laser is helium-neon laser. The helium-neon laser operates by electrically exciting a mixture of helium and neon gas. This excitation creates photo emission, thus producing optical signals. The optical signals are caused to resonate within the aforementioned cavity by placing highly reflective mirrors at the comers to create a closed path. Due to the arrangement of electrodes and the design of the cavity, two optical signals are created, each propagating in opposite directions around the closed loop path.
At one comer of the block there is positioned a partially transmissive mirror which allows a portion of the optical signals to escape from the cavity while the remainder of the optical signals are reflected. This mirror is commonly known as the output mirror because of its having the appropriate output sensors associated therewith. In operation, a portion of each optical signal is allowed to escape from the cavity and subsequently be combined with the other optical signal to sense the aforementioned frequency separation.
The amount of transmission and the amount of reflection are chosen for the output mirror based upon the necessary signal strengths and laser power levels.
Due to the criticality of the two optical signals, it is very important to create a very stable closed loop path. Generally this is accomplished by controlling the thermal expansion characteristics of the block and all related elements (e.g., the mirrors). Materials are chosen for the block which have very low coefficients of thermal expansion, such as Zerodur (a glass produced by Schott Glass Technologies of Duryea, Pa.) or BK-7 (also produced by Schott Glass Technologies of Duryea, Pa.). Similarly, the mirrors must be produced to be very stable over temperature changes.
In the operation of the ring laser gyroscope, the performance of the mirrors creating the closed loop optical path is critical. As previously mentioned, one of these mirrors is the output mirror which must have very stable optical characteristics. Included in the characteristics for the output mirror are reflectance/transmission ratios, the scattering characteristics, and degradation over time.
In addition to the necessary optical characteristics, the output mirror must also meet numerous other design attributes. First, the output mirror must have thermal expansion characteristics which closely match the thermal characteristics of the block. This thermal matching avoids errors in the gyro due to incompatible thermal expansion. Secondly, the output mirror must be capable of being sealed to the block so as to form a gas tight seal. Lastly, the output mirror must have the necessary Helium diffusion characteristic. As previously mentioned, the laser is created using a mixture of helium and neon within a closed loop cavity. To insure consistent operation over time, the cavity must be capable of maintaining constant gas mixtures and pressures. This is a concern because of the knowledge that certain materials allow helium gas to escape through the materials lattice structure. Therefore, it is necessary to utilize materials that have very low helium diffusion characteristics. Examples of material used as substrates in the construction of ring laser gyro output mirrors includes glasses (Zerodur and BK-7), and fused silica quartz. Many of the aforementioned characteristics are desirable for all mirrors on the gyroscope, however the present invention is primarily concerned with the output mirrors only.
By meeting all of the aforementioned, the gyro is assured to operate very accurately, assuming all other necessary systems are operating effectively. Other systems are well known in the art and are beyond the scope of the present invention.
Complications arise, however, when the gyroscope is operated in a radiation environment. The exposure to radiation is a concern since the ring laser gyroscope is used in natural space, weapons enhanced space, strategic and tactical weapons environments. Radiation has numerous effects on the operation of the gyro, some of which are detrimental to the gyro's operation. More specifically, radiation causes the darkening of numerous glass type materials, including the darkening of the output mirrors as they are exposed to radiation. The net result of the exposure to radiation, generally speaking, is a lowering of the optical characteristics of the mirrors. More specifically, the transmission characteristics of the glass materials is greatly reduced causing a reduction in the strength of the signals being transmitted by the mirror to the output sensors.
As previously discussed, the characteristics of the output mirror is critical to the operation of the gyroscope, therefore it is necessary to make alterations and adjustments to the output mirror to accommodate operation in radiation environments. Currently, there does not exist a output mirror that will withstand the levels of radiation expected while also demonstrating the attributes necessary for optimum gyroscope performance.
Another problem that affects the operation of the output mirror is the glow that exists in the closed loop cavity of the ring laser gyroscope. The glow is a by-product created by the gas discharge within the cavity. Glow is undesirable since it creates a DC offset component in the output signals. This DC component corrupts the true laser power and the output signal processing and represents a false reading to the output sensors. As a result, the dynamic range of the output signal processing is reduced along with a reduction of the useful input voltage range for the signals.